There are no planets between Mars and Jupiter, just a band of debris called the asteroid belt. Most are just a few metres in length, but the largest asteroid, Ceres, is 945km in diameter and is often classified as a dwarf planet. NASA's space probe Dawnhas recently started orbiting Ceres. Asteroid impact is currently the best theory for why the dinosaurs died out 65 million years ago. The 1998 Bruce Willis movie Armageddon fuelled concerns that a near-Earth asteroid, such as Apophis, might collide with the Earth again.